1. Field of the Invention
The present invention relates to a thixocasting semi-molten casting material, and a process for producing the same.
2. Description of the Prior Art
In carrying out a thixocasting process, a procedure is employed which involves subjecting a casting material to a heating treatment to produce a semi-molten casting material having a solid phase (a substantially solid phase and this term will also be applied hereinafter) and a liquid phase coexisting therein, charging the semi-molten casting material into a cavity in a casting mold under a pressure, and solidifying the semi-molten casting material under the pressure.
In the heating treatment, the solid phase content in the semi-molten casting material is set such that the thixocasting process is smoothly conducted. Under such a solid phase content, the flow resistance to the semi-molten casting material is reduced and hence, the following disadvantage is liable to arise: a portion of the semi-molten casting material flows out, or the semi-molten casting material is deformed.
An approach employed in the prior art is to fit the casting material with a ring prior to the heating treatment to prevent the flow-out and the deformation by the ring (for example, see U.S. Pat. No. 4,712,413).
However, the prior art approach is accompanied by a problem of the need for operations including the fitting of the casting material with the ring, the detachment of the ring from the casting material and the removal of a solidified metal portion deposited to the ring, resulting in a complicated casting procedure.
From the metallographic and economic viewpoints, the casting material has been produced generally by utilizing an agitated continuous casting process, but in the process for producing the casting material, it is not avoided that an outer layer portion having dendrites exists around an outer periphery of a main body portion of the casting material. The dendrites cause the pressure for charging the semi-molten casting material into the cavity to rise to impede the complete charging of the semi-molten casting material into the cavity, and hence, the dendrites are useless in the casting material.
Therefore, following approaches have been conventionally employed: an approach to remove the dendrites by a dendrite trap mounted in a casting mold (see Japanese Patent Publication No. 51703/90, and an approach to subject the casting material to a cutting to remove the outer layer portion.
However, the former approach for removing the dendrites by the dendrite trap mounted in the casting mold causes the structure of the casting mold to be complicated, and brings about an increase in cost. The latter approach to cut off the outer layer portion brings about an increases in process steps and a deterioration of productivity.